When opened in 1890, the line had six stations and ran for 3.2 miles (5.1 km)[2] in a pair of tunnels between the City of London and Stockwell, passing under the River Thames.

The diameter of the tunnels restricted the size of the trains, and the small carriages with their high-backed seating were nicknamed padded cells.

In November 1883, notice was given that a private bill was to be presented to Parliament for the construction of the City of London & Southwark Subway (CL&SS).

[5] Before the railway opened, a further bill received assent, granting permission to continue the line south to Clapham Common.

[16] The depot was on the surface, and trains requiring maintenance were initially hauled up via a ramp although, following a runaway accident, a lift was soon installed.

To avoid the need to purchase agreements for running under surface buildings, the tunnels were bored underneath public roads, where construction could be carried out without charge.

Business men who have great distances to come will by this means find an easy way of leaving the City and of enjoying the fresh air of the country.

All classes of the community are obliged to travel at the same fare, which is the small sum of twopence, and are by a very simple arrangement able to save a great deal of time and trouble.

I have been immensely struck by what I have seen today, and I am sure that the greatest credit redounds upon those who have contrived this scheme, and have carried it to such perfection...The original service was operated by trains composed of an engine and three carriages.

Thirty-two passengers could be accommodated in each carriage,[16] which had longitudinal bench seating and sliding doors at the ends, leading onto a platform for boarding and alighting.

[16] Despite the cramped carriages and competition from bus and tram services, the railway attracted 5.1 million passengers in 1891, its first year of operation.

[25] Shortly before it opened to the public, the C&SLR gave notice of its intention to submit another private Bill to Parliament, to construct a new line from its northern terminus at King William Street towards Islington.

[26] Because of the awkward arrangement of King William Street station, the extension was not to be connected directly to the existing running tunnels but was to be linked via a pedestrian subway through which passengers could make interchanges between the separate lines.

[28] The company had recognised the deficiencies of its King William Street station and, just a year after the line had opened, planned to construct a new pair of tunnels to bypass the problematic northern section.

c. v), approved on 25 May 1900,[40] gave permission to enlarge the station tunnel at Angel to a diameter of 9.2 m (30 ft 2 in)[41] [42] and the rest of the extension opened on 17 November 1901,[43] with stations at: Despite the technical innovations of the railway and the large passenger demand,[25] the C&SLR was not particularly profitable and the rapid series of extensions aimed at improving profits had placed a strain on the finances.

[44] In an attempt to work around this poor reputation and make it easier to raise funds, the next Bill for an extension of the line was submitted in November 1900 by a notionally separate company, the Islington and Euston Railway (I&ER),[45] albeit one that shared its chairman with the C&SLR.

[50] In November 1902, the C&SLR submitted a Bill in its own name for the Euston extension as well as the authority to take over the dormant powers of the C&BR.

[52] This time, the Bill was approved and received royal assent as the City and South London Railway Act 1903 (3 Edw.

[56] A separate Bill was published at the same time by the London Electric Railway (LER, a company formed by the UERL in 1910 through a merger of the BS&WR, GNP&BR and CCE&HR), which included plans to construct tunnels to connect the C&SLR at Euston to the CCE&HR's station at Camden Town.

[57] Together, the works proposed in these Bills would enable the CCE&HR's trains to run over the C&SLR's route and vice versa, effectively combining the two separate railways.

In February 1919, with the war over, the C&SLR submitted a new Bill that included provisions for an extension of time for the tunnel enlargement works approved in the Act of 1913.

[63] The projects were made possible when the government introduced the Trade Facilities Act 1921 by which the Treasury underwrote loans for public works as a means of alleviating unemployment.

The northern section of the C&SLR between Euston and Moorgate was closed from 8 August 1922, but the rest of the line remained open with enlargement works taking place at night.

[64] A collapse on 27 November 1923 caused when a train hit temporary shoring on the incomplete excavations near Elephant & Castle station filled the tunnel with soil.

While the reconstruction works were underway, the C&SLR submitted a Bill in 1922 that contained proposals to extend the line south from Clapham Common through Balham and Tooting to Morden in tunnel.

Thus fully integrated, combined services operated over the C&SLR and CCE&HR routes using new Standard Stock trains.

In an effort to protect the Group's income, its managing director/Chairman, Lord Ashfield, lobbied the government for regulation of transport services in the London area.

During the 1920s a series of legislative initiatives was made in this direction, with Ashfield and Labour London County Councillor (later MP) Herbert Morrison, at the forefront of debates as to the level of regulation and public control under which transport services should be brought.

Ashfield aimed for regulation that would give the existing Group protection from competition and allow it to take substantive control of the LCC's tram system; Morrison preferred full public ownership.

[75] The technologies of deep tube tunnelling and electric traction pioneered and proved by the C&SLR shaped the direction of subsequent underground railways built in London.